1. Field of the Invention
The present invention relates to a high pressure metal vapor discharge lamp, more specifically, relates to a small size high pressure metal vapor discharge lamp of 100W or less.
2. Discussion of Background
Generally, incandescent lamps are used for the light source for vehicle headlights. However, incandescent lamps have problems such as the facts that their light emission efficiency or efficacy is low and they have a short life, which means that the lamps have to be replaced frequently. As opposed to these, discharge lamps, which have high efficacy and a long life, are known as light sources. For example, fluorescent lamps which are low pressure discharge lamps, are used as lamps inside buses or electric trains. However it has not been possible to use fluorescent lamps as light sources for headlights since they would be too large. In view of this situation, there have been attempts at technical development to produce headlight light sources in the form of high pressure metal vapor discharge lamps, e.g., metal halide lamps or high pressure sodium lamps, which have a higher efficacy than fluorescent lamps and can easily be made compact. When such a discharge lamp is used, in view of aspects such as the size of the headlights, the required light intensity and consumption of the vehicle's batteries, etc., it is preferable to have a discharge lamp with a electricity consumption of 10W (watts) or less. However, one problem when a small size high pressure metal vapor discharge lamp such as this, e.g., a small size halide lamp, is used as a light source for headlights is taking long time for rising the lamp's luminous output. That is, on startingup of the lamp, there is hardly any vaporization of the mercury or metal halide sealed in the arc tube immediately after start-up and so there is only 10% luminous output at most of the lamp brightness that obtains rated operation. It usually takes 3-10 minutes for the arc tube to reach a high temperature and come into a stable lighting state and even if heat-holding effects are improved or the current at the time of start-up is made greater, the rise up time is still 30 seconds-1 minute, which makes practical applications difficult.
A way one can think of for resolving this problem is a system for starting an arc tube by effecting preheating with a heater, etc. For example, the publication of Japanese Laid-open Patent Application No. 51-4881 discloses a metal halide lamp wherein a guide for a heater is provided in the vicinity of the coldest portion of an arc tube and quartz wool is packed between the arc tube's coldest portion and the guide as a heat resisting electrical insulator. The object of this invention is to control the lamp's color temperature within a required range by adjusting the electric current in the heater coil, and whereby the heater coil temperature is changed and the temperature of the arc tube's coldest portion is controled arbitrarily from the exterior. And the invention can also be thought to be connected with improvement of the rise time so as to take short time, the problem noted above. However, since the heater coil is exposed inside an outer tube in a means such as this, depending on the height of pulses imposed at the time of lamp ignition, discharge between the heater coil and the arc tube's lead wires may occur inside the outer tube, so resulting in failure for sufficient pulse energy to be supplied to the lamp, and there is therefore a risk of start-up being uncertain. Also, since there is packing of quartz wool as described above between the arc tube and the heater coil, when the lamp is lit and preheating power is no longer supplied to the heater coil, the heat of the arc tube escapes to the exterior, transmitted by the contacting packing and heater coil. Therefore, there are the drawbacks that the heat-retention effects of the arc tube actually become lower, the efficacy is lower because of lowering of the vapor pressure by material sealed in the arc tube and a required emitted light color fails to be produced. Avoiding this situation demands that heater power be provided in addition to lamp power, since the heat conduction loss from the arc tube to the heater must be suppressed by supplying power to the heater coil even when the lamp is stably lit, and so a means such as this is in no way permissable if one considers the amount of consumption of vehicle batteries.